In recent years, non-woven fabrics comprising synthetic fibers, by making the best use of characteristics of those fibers, are extending their applications into various fields, such as clothing materials, medical materials, engineering and building materials, and materials for industrial use.
Among them, non-woven fabrics containing PTFE fibers are excellent in heat resistance, chemical resistance and abrasion resistance, and are expected to be further developed as highly functional non-woven fabrics.
Cotton-like PTFE materials being made into the non-woven fabrics are gathered PTFE fibers, and so far have been made in such manners as mentioned below:
(1) A process for producing filaments and then cutting to a desired length. PA1 (1a) An emulsion spinning method disclosed U.S. Pat. No. 2,772,444. This method comprises extrusion spinning of a viscose binder, and the like containing PTFE particles, and then sintering to obtain the filaments having a circular section. Major problems of that method are such that a binder remains as a carbonaceous residual after sintering, the obtained PTFE filaments are colored in a dark brown, and that even if the carbonaceous residual is oxidized to be discolored, an original purity cannot be maintained. PA1 (1b) A method disclosed in JP-B-22915/1961 or JP-B-8769/1973. This method comprises stretching of fibers obtained by slitting a PTFE film to a desired width. A problem of this method is that the smaller the slit width is, the more easily the fibers are broken at the time of stretching. PA1 (2) A process for preparing PTFE fibrous powder in the form of a pulp and making a sheet-like material therefrom by paper making process (U.S. Pat. No. 3,003,912 and JP-B-15906/1969).
The process for producing PTFE filaments is roughly classified into the following two processes.
Both PTFE fibers obtained by the methods (1a) and (1b) have a low friction coefficient and a high specific gravity inherent to the PTFE, and therefore are not intermingled sufficiently with each other even if having been crimped. (JP-B-22621/1975)
The method of the above-mentioned U.S. patent is to cut PTFE rod obtained by a paste extrusion, to a short length and to apply a shearing force to obtain fibrous PTFE powder.
JP-B-15906/1969 discloses a method for making fibers by applying a shearing force to the PTFE powder.
Any of the fibrous powder obtained by the above-mentioned methods can be made up to a sheet-like material by paper making process but cannot be made into a non-woven fibric by the use of a carding machine, needle punching machine, or the like as they are short in fiber length and in the form of a pulp.
An object of the present invention is to provide the PTFE fibers excellent in intermingling property and cotton-like materials containing those fibers.
Another object of the present invention is to provide a process for obtaining cotton-like PTFE materials, which are staple fibers (relatively short fibers), directly from a uniaxially stretched long film of PTFE, without making multi-filaments (a large number of continuous fibers).